(The person or group that made the initial submission then get a 1% royalty of net sales)
[I'm not connected to the groups submitting the Portal sets... I just happen to really like both LEGO and Portal]
Cuusoo seems to host the platform and I assume (but don’t know) that they take care of all technical issues, Lego seems to take care of the review process. So Lego Cuusoo is basically the Cuusoo installation run by Lego.
You are completely correct about the review process, though (basically everything you wrote after the first sentence).
Also, sometimes the legos will end up less promising than it seemed at the beginning - see the official Minecraft Cuusoo lego - http://www.jinx.com/p/lego_minecraft_micro_world.html .
And also, the LEGOs are not produced in so big numbers as normal lego boxes, so they are not sold in regular retail, but only on internet in limited numbers.